This invention relates to overcurrent and overtemperature protection of power transistor switches.
Many power control applications use power transistors as switches. For example, in the data processing area power transistors are used in the switching mode to turn on and off display lights and to activate solenoid drivers used in printers and key punches. Power transistors are also used in the switching mode as solenoid drivers in a wide variety of applications such as combines and other agricultural implements. In the instrumentation area power transistors are operated in the switching mode for all sorts of voltage and current regulated power supplies, and in areas where portable AC is needed, switching mode power transistor circuits convert DC from batteries to AC at desired frequency. In electrically driven vehicles and in electric hand tools, transistors used in the switching mode chop battery current to control energy flow from the battery to the electric motor.
In many such applications of power transistors as switches, a fault condition results in desaturation and excessively high current which causes rapid thermal destruction of the switching transistor. Failure of the power transistor necessitates its replacement before the apparatus can be used. For example, if personnel testing an agricultural combine having solenoid circuits negligently touch a metallic tool to terminals which results in short circuit of the solenoid coil, a fault current of high enough magnitude may flow and cause the driver transistor to come out of saturation and destroy the transistor. As a result, the combine cannot be used until the failed drive transistor is replaced.
In general, fuses do not provide effective protection for such switching transistor because, unless the transistor is of considerably greater current rating than required for the application, the transistor will usually be thermally destroyed before the fuse ruptures. Current threshold sensing circuits are known wherein the switching transistor normally conducts in the saturated condition, and if the switching transistor starts to come out of saturation, feedback through an on-off control circuit gates the base current to the switching transistor off to prevent transistor failure. Base current is gated on again by a trigger pulse at the leading edge of each switching cycle, and the switching transistor remains on as long as it is conducting in the saturated condition. Such a current threshold sensing and conduction limit circuit for motor control is disclosed in U.S. Pat. No. 3,855,520 to F. A. Stich having the same assignee as this invention, but such circuit is too costly and complicated for most power control applications.